It's often said that you can't win the Presidency with a single debate performance, but you can lose it.
So will we look back on last night and say that was the moment that Trump finally lost his outlandish bid for the White House?
There are 42 days to go, and two more debates. So it's too early to say Trump can't recover.
And we know not to underestimate the Republican candidate, who has defied political gravity all year.
But let's call this debate for what it was. It was a Hillary Clinton victory.
Decisive and unambiguous. On every level - temperament, policy knowledge, political skill, emotional stability, stamina - she won.
Donald Trump appeared dreadfully unprepared; and he couldn't hide it in the fiery one-on-one, 90-minute prime time encounter.
Some of his comments were, frankly, bizarre.
At one point, Trump was being openly mocked in the media centre, with US reporters laughing at his clumsy, awkward replies.
Maybe that doesn't matter either, for this election will be won not among media elites but in the heartland, in swing states like Pennsylvania, Ohio and Wisconsin.
The question remains: How did Trump go down with undecided voters in these key regions?
We don't yet know; the polling will take a few days to factor the debate in.
The polls currently show this race to be very close, maybe even a dead heat.
But something changed here overnight.
In the hour before the debate I detected real fear among Hillary's top surrogates.
By the time it was over, there was another, equally powerful emotion. Relief.